Lecture #10 notes; Geology ; CR Stern Volcanic landforms - Part I (pages in the 5 th edition)

Transcription

1 Lecture #10 notes; Geology ; CR Stern Volcanic landforms - Part I (pages in the 5 th edition) The different types of magmas generated in different types of tectonic environments create different types of volcanoes or volcanic landforms. By looking at a volcano, one can gain an impression of what type of magma it erupts and whether or not it produces passive eruptions of lava flows or explosive eruption and pyroclastic flows, as summarized in this table from the text

2 Eruption of basaltic magmas along mid-oceanic extensional plate boundaries and continental rift systems, or above hot-spots, produce 1) lavas plateaus from large volume fissure eruptions as occur in Iceland, where the largest historic eruption of basaltic lava, the Laki eruption, which produced 12 km 3 of basaltic lava from a 25 km long fissure over a 9 month eruption in 1783, or the Columbia plateau basalts in the western USA which formed between 16 to 15 million years ago when the hot-spot that is now under Yellowstone park first appeared at the earths surface (see figures 1 and 2 below). In the Columbia plateau basalts, single flows have volumes >1500 km 3, or 1000X bigger than the Laki eruption of 1783 (figure 3 below) Figure 1 the Colombia plateau basalts formed Ma above a hot-spot now below Yellowstone park Figure 2 The Columbia Plateau basalts in Southeastern Oregon

3 Figure 3 the Roza flow, a single basaltic lava flow with a volume of 1500 km 3 in the Columbia plateau basalts Lava plateaus are forming today in the Rio Grande rift valley, part of the extension rift tectonic environment in the western US in southern Colorado and northern New Mexico by Taos (figures 4 and 5) west of the Rocky Mountains. Figures 4 and 5 plateau basalts and small shield volcanoes cut by the Rio Grand river in New Mexico. The bridge over the river is near Taos

4 2) large volume eruptions of basalts from central vent volcanoes form shield volcanoes with low angle sides dues to the low viscosity of the basaltic magma eruption and the long distance it flows away from the source. Shield volcanoes are characteristic of Hawaii (figure 6) and other oceanic hot-spot islands such as the Galapagos islands (figure 7) Figures 6 and 7 The shield volcanoes that make up the large island of Hawaii and the Galapagos islands, both of which are oceanic hot spots which erupt basalt

5 3) small volume basaltic eruptions produce scoria cones and lava flows, such as occur all over the western US in the Basin and Range extensional volcanic province and on the Colorado plateau from New Mexico as far west as eastern California (figures 8, 9 and 10 below). Scoria cones form from fire fountains of basaltic lavas caused by escape of gas from the magma, but basalts do not have enough gas to produce explosive eruptions. Figures 8-10 Small volume basaltic scoria cones and lava flows at Devils Throne on the north rim of the Grande canyon, near Flagstaff AZ south of the canyon, and in Dotsero, Colorado along I-70 just east of Glenwood canyon. All these are very young geologically, all <5,000 years old, and form by extension and rifting.

6 Eruption of andesitic and rhyolitic magmas in subduction generated convergent plate boundary volcanic arcs, and/or in continental rifts and hot-spots where mantle derived basalts (see lecture #9 figures 1 and 4) interact with the continental crust and become andesites and rhyolites, form: 1) stratovolcanoes (figures 11-13), or composite volcanoes, which are steep sided because of the viscous nature of the cooler andesitic and rhyolitic magmas that are erupted in this tectonic environment Figures are three circum-pacific rim of fire stratovolcanoes (Mt Rainer in the Cascades, Mt Fuji in Japan and Mt Maca in the Chilean Andes) above subduction zones around the Pacific Ocean. Note there steep sides compared to shield volcanoes, due to the higher viscosity of the andesite magmas they erupt compared to basalts. All these volcanoes are potentially explosive like Mt St Helens or could form calderas like Crater Lake as described below.

7 2) Calderas, like Crater Lake in the Cascade mountains (figure 14) result when the tops of stratovolcanoes collapse into the empty space of a magma chamber after a large volume explosive eruption (figure 15). caldera formation depends on the volume of magma erupted. The Mt St Helens eruption of 1980, which erupted 1 km3 of magma (Figure 16), was not large enough to produce a caldera, only a summit crater (see Lecture #9), but the 79AD eruption of Vesuvius that covered Pompii (figure 17), the 1883 eruption of Krakatoa, the 1550 BC eruption of Santorini in the Mediterannean sea (figure 18) and the 4400 BC eruption of Mt Mazama to form Crater Lake (figure 15) were all large enough to generate calderas, and all have experienced subsequent eruptions now rebuilding their original stratovolcano forms.

8 Mt Vesuvius, Italy, is a stratovolcano inside the Somma Caldera the wall of which is the small bump on the left of Vesuvius. This caldera formed in the 79AD eruption that covered Pompii, and the new Vesuvius cone has been built up since then. This volcano results from the subduction of Mediterranean oceanic crust below Italy due to the northward drift of Africa, the same process that has generated the Alps mountains. Santorini, a caldera in the Greek islands, formed by a large eruption at about 1550 BC. Like Krakatoa, the collapse of the volcano must have caused tidal waves as water entered the caldera, as well as tephra falls all over the eastern Mediterranean area. This eruption has been related to the myth of the disappearance of Altlantis. Note the new volcanic island forming in the center of the caldera over the last 3500 years.